1. Field of the Invention
The invention is concerned with production line quality control of semiconductor junction devices. The category of applicable devices includes those containing homojunctions, heterojunctions, those in which the junction is formed between semiconductor materials, semiconductors and metals, as well as semiconductors and ionic liquids.
2. Description of the Prior Art
The applicability of semiconductor junction devices for many uses was perceived as a value virtually with the inception of such devices. The earliest literature on germanium and silicon homojunction single crystal devices alludes to photocurrents and to the possibility of using such structures as primary power supplies. In fact, commercial fruition of single crystal photocells, generally in relatively low power devices, was attained some years ago; and relatively sophisticated circuitry requiring such low power, for example, in satellite-born equipment, has been used for some time.
Early and continued interest in the possible use of junction devices as primary power supplies at higher power levels is evidenced by the numerous articles on this topic published in a variety of technical journals over the years. This interest has recently been intensified by general concern with the future availability of fossil fuel. Visualizing the desirability of responsive, large area devices, intensified interest has been directed toward polycrystalline rather than single crystal devices. While such polycrystalline devices may depend upon homojunctions, improved flexibility in device operation (e.g., tailored transparency to incident radiation on the irradiated side of a junction with efficient absorption on the other), and other considerations, including some concerned with manufacturing expediency, have largely dictated the use of heterojunctions. Heterojunctions of interest have taken the form of semiconductor-semiconductor structures and, perhaps, most significantly, of semiconductor-liquid junction structures (see Journal of Electroanalytical Chemistry, 58 (1975) 263 by H. Gerischer).
At this time, it is generally accepted by workers in the field that the large area devices contemplated for generation of significant amounts of power will necessarily contemplate use of polycrystalline layers. To date, the various polycrystalline devices reported share an operating deficiency--a marked decrease in photo conversion efficiency as compared with single crystal homojunction devices. Typically, polycrystalline devices reported have operated at efficiencies less than several percent as compared with the ten to fourteen percent range regularly attainable in a traditional silicon solar cell.
It is generally recognized that a cause of the relative inefficiency of polycrystalline devices as compared to single crystal devices is the recombination of photogenerated carriers at grain boundaries or structural defects. Some trapping mechanisms are, as recognized, avoidable by techniques such as appropriate material choices, properly engineered fabrication, etc. Other trapping mechanisms are concerned with factors such as simple lattice mismatches, differences in thermal expansion, contamination, etc., which are avoidable by conventional approaches. Appropriate material selection and processing steps, such as surface etching, have resulted in improved efficiency. It has become evident, however, that the effective processing expedients contribute to the complexity of fabrication and create a need for accurate quality control.
The trapping mechanisms discussed above are, of course, of concern in junction devices generally regardless of the intended purpose of such devices and quality control procedures which may be usefully employed as a basis for monitoring processing conditions to insure minimization of trapping centers are therefore of general interest.